Abstract:

The effects of high-valent inorganic salts on the surface activity and phase behavior of anionic surfactant sodium lauroyl methyl taurate （or sodium methyl lauroyl taurate, SMLT） were studied. The effects of Ca²⁺ concentration on the surface tension of SMLT solution and the effects of different SMLT/Ca²⁺ molar ratios on the phase behavior of SMLT in water were investigated. The results showed that the addition of an appropriate amount of Ca²⁺ had significant impact on the surface activity. When the Ca²⁺ concentration was increased from 0 to 0.1 mol/L, the cmc decreased from 8.0×10^-3 mol/L to 8.0×10^-4 mol/L, and the γ_cmc decreased from 36.00 mN/m to 31.45 mN/m. Meanwhile, by adding Ca²⁺ to the SMLT solution, the binary phase diagram of the SMLT/water system showed phase regions of micellar solution region, wormlike micelle region, hexagonal liquid crystal region, lamellar liquid crystal region, and micelle-crystal region. The original aqueous two-phase region disappeared, and the micelle-crystal region became larger. When examining the effect of Ca²⁺ concentration on the viscosity of SMLT solution, it was found that, with a molar ratio of SMLT/Ca²⁺ of 1, the viscosity was significantly increased, while at a molar ratio of 0.6, solid precipitated from the solution. An SMLT solution at mass fraction of 20% was selected for measurement of rheological performance. According to the rheological curve of the sample, a phenomenon of shear thinning was observed, and the formation of wormlike micelles was observed by cryogenic transmission electron microscopy.

Key words: inorganic salt, surfactant, phase behavior, worm-like micelle